48x21 shouldn't be a problem.  Might get a little close to the monitor if you are adding a trackball but otherwise no problem.  My last 2 CP's are 12" total depth even with a trackball and while I am not going to be swinging for the long drives in golden tee, it works just fine for everything else.  Width is your only enemy with 4 player CP's, you don't want to be pushing your other players away from their controls.  48" is more than enough.